1. Field of the Invention:
The present invention relates to apparatus for the remote inspection of the interior surfaces of tubes. The invention has particular application to the maintenance of steam generators, particularly nuclear power plant steam generators.
2. Descritpion of the Prior Art:
A nuclear steam generator contains many vertical tubes arranged in close relationship to each other. A primary fluid, having been heated by circulation though the nuclear reactor core, is circulated through the tubes. At the same time, a secondary fluid, or feedwater, is circulated around the tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes, causing a portion of the secondary fluid to be converted to steam.
Isolation of the radioactive primary fluid from the secondary fluid in a nuclear steam generator is critical. Accordingly, the integrity of the heat exchanger tubes is also critical, and test and maintenance procedures have been developed for inspection of the tubes and the correction of defects therein. One technique for correcting local defects in the tubes is sleeving, i.e., mounting an auxiliary tube section inside the defective tube to span the defective region, thereby returning the tube to its normal heat transfer capacity.
Fundamental to both the inspection and maintenance procedures is the insertion of probe devices into the tubes for detecting defective regions and for inspecting the repairs. One such inspection is conducted during the sleeving operation. The sleeves are brazed or welded to the interior surfaces of the tubes, and these surfaces, which typically have an accumulation of oxidation, must be honed (or wire brushed) in preparation for brazing or welding. After the honing, the surface must be inspected to be sure that it is in proper condition for brazing, and this necessitates that the inspection probe be able to distinguish between the bright or shiny honed surface and the surrounding dull or oxidized surface of the tube.
Optical inspection techniques have been used for directly viewing or scanning the interior surface of the tube, as with fiber optic probes. Such probes may involve transmitting light into the tube from a remote light source through a long fiber optic cable, the reflected light being returned through another portion of the cable to a camera or other viewing means at the remote location. But detection capability may be restricted with such systems due to the optical losses in the long fiber optic cables or as a result of inspecting light-absorbent or dark surfaces, such as those coated with oxide deposits, which require higher levels of illumination. Furthermore, such fiberscopes are very expensive, require delicate handling, are subject to easy breakage, and must be rotatable to obtain complete circumferential coverage of a tube. Also, such optical imaging systems rely on the subjective determination of the individual inspector, who bases his determination solely on what he sees. Thus, the results are dependent on the competence and visual acuity of the operator, making standardization difficult, if not impossible. Furthermore, the measurements from such optical imaging systems are degraded by the effects of ambient light.
It is known to minimize the problem of optical losses by mounting the light source and the light detection means on the probe, so that only electrical signals are transmited over the long cable which extends to the remote control location. But such systems are still subject to the other disadvantages of optical imaging systems, discussed above.
While the present invention relates principally to the inspection of the interior surfaces of tubes in preparation for sleeving, it also would have more general application for in-service inspection of steam generator tubing, e.g., for defect detection, when a quick diagnosis of tube condition is required.
Various inspection techniques have been used heretofore for such purposes, including ultrasonic and eddy current techniques. Ultrasonic techniques have the disadvantage that the ultrasonic energy must be mechanically coupled into the tube wall, as by a liquid coupling medium. Eddy current techniques are only usable with certain materials, and the measurements are affected by the presence of structures, such as support plates, external to the tube. Furthermore, such prior techniques have typically entailed subjective operator interpretation of data, such as comparison of the data with standard profiles, and this leads to inaccuracies and inconsistencies in the results, since it is dependent upon individual operator skill.